Method of making a semiconductor laser with a self-sustained pulsation

ABSTRACT

A semiconductor laser with a self-sustained pulsation is disclosed in which a first cladding layer of first conductive type, an active layer and a second cladding layer of second conductive type having a striped ridge are formed in that order on a semiconductor substrate of first conductive type. The first and second cladding layers have a refractive index smaller than and a band gap larger than the active layer. A saturable optical absorbing layer having a band gap of energy substantially equal to the energy corresponding to lasing wavelength is formed in both the first and second cladding layers. Further, a barrier layer having a refractive index smaller than and a band gap larger than the first and second cladding layers is formed between the first cladding layer and the active layer and/or between the active layer and the second cladding layer.

This is a division, of application Ser. No. 08/147,779, filed Nov. 4,1993, now U.S. Pat. No. 5,416,790.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser with aself-sustained pulsation in which the noises attributable to the opticalfeedback of a laser beam are reduced.

2. Description of Related Art

The problem of conventional semiconductor lasers is that in the casewhere the optical feedback of a laser beam is applied repeatedly, anoise attributable to the optical feedback (called the optical feedbackinduced noise) is generated in the laser beam. The optical feedbackinduced noise is generated in such a manner that the optical feedback ofthe laser beam due to the reflection from the disk surface or the likereenters the semiconductor laser when it is used, for example, as alight source of an optical disk system.

An approach to a reduced optical feedback induced noise of thesemiconductor laser by utilizing the self-sustained pulsation isdisclosed by Japanese Patent Application Laid-Open No.63-202083(1988),for example. In this semiconductor laser, one of cladding layerssandwiching an active layer is constructed of a refractive index layerhaving a band gap energy considerably larger than the energycorresponding to the lasing wavelength (energy corresponding to lasingwavelength: hν, where h is Planck's constant and ν frequency) or anoptical absorbing layer having a band gap energy considerably smallerthan the energy corresponding to lasing wavelength to generate aself-sustained pulsation.

According to the above-mentioned conventional semiconductor laser,however, in the case where a refractive index layer is used with a bandgap energy considerably larger than the energy corresponding to lasingwavelength, the astigmatic distance is increased, while the use of anoptical absorbing layer with a band gap energy considerably smaller thanthe energy corresponding to lasing wavelength increases the operatingcurrent. With a large astigmatic distance, the laser beam is difficultto converge. In the case where the semiconductor laser is used as alight source for an optical disk system or the like, therefore, the S/Nfor crosstalks is deteriorated, and an increased operating currentincreases the power consumption.

Japanese Patent Application Laid-Open No. 61-84891 (1986), on the otherhand, discloses a semiconductor laser having a thin film layer(saturable optical absorbing layer) formed only in one of the claddinglayers, which has substantially the same composition as the active layerand a saturable optical absorption characteristic against a laser beam.Such a saturable optical absorbing layer can be easily formed by crystalgrowth, thereby making it possible to manufacture a semiconductor laserwith a high volume productivity.

Nevertheless, experiments conducted by the present inventors haverevealed that in the case of the above-mentioned conventionalsemiconductor laser, it is difficult to reduce the astigmatic distancesufficiently only by the relation between the composition ratio of thesaturable optical absorbing layer and that of the active layerdescribed.

If a high-output semiconductor laser is to be realized, a desiredstructure includes a first cladding layer, an active layer and a secondcladding layer having a ridge section. This structure makes it necessaryto control the film thickness of the flat portion of the second claddinglayer at high accuracy. Japanese Journal of Applied Physics Vol.25,No.6, June 1986, pp. L498-L500, for example, discloses a semiconductorlaser including a compound semiconductor containing Al in which anetching stop layer having an Al composition ratio different from thesecond cladding layer and absorbing no pulsated laser beam is formed onthe flat portion to control the thickness of the flat portion.

Also, Nikkei Electronics, Aug. 31, 1981, pp.76-79 presents asemiconductor laser in which a very thin barrier layer for enclosing thecarriers strongly without substantially affecting the light is formed onboth sides of the active layer, and light is enclosed not by therefractive index difference between the active layer and the barrierlayer but by the index difference between the active layer and thecladding layers thereby to improve the output beam divergenceperpendicular to the junction. The disadvantage of this semiconductorlaser is that thickness variations of the thin barrier layer have agreat effect on the laser characteristics. It is therefore necessary tocontrol the film thickness at high accuracy in order to avoid a lowproduction yield.

SUMMARY OF THE INVENTION

An object of the invention is to provide a semiconductor laser forgenerating a self-sustained pulsation with a small astigmatic distanceand a small operating current.

Another object of the invention is to provide a semiconductor laser witha self-sustained pulsation having a superior operating currentcharacteristic and a superior beam divergence characteristicperpendicular to the junction.

Still another object of the invention is to provide a semiconductorlaser with a self-sustained pulsation in which band gap energy can beeasily matched between a saturable optical absorbing layer and an activelayer thereby making it difficult to produce a kink at low opticaloutput power.

A semiconductor laser with a self-sustained pulsation according to theinvention has a first cladding layer of first conduction type formed ona semiconductor substrate, an active layer formed on the first claddinglayer and a second cladding layer of second conduction type having aridge section on the active layer, wherein the first and second claddinglayers have a small refractive index and a large band gap as comparedwith the active layer, and a saturable optical absorbing layer is formedwith a band gap of an energy substantially equal to the energycorresponding to lasing wavelength in both the first and second claddinglayers. The fact that a saturable optical absorbing layer having a bandgap energy substantially equal to the energy corresponding to lasingwavelength is formed in the first and second cladding layers sandwichingthe active layer causes a self-sustained pulsation with a low astigmaticdistance and a low operating current in unit transverse mode. Also, thesaturable optical absorbing layer in the second cladding layer functionsas an etching stop layer in forming the ridge section of the secondcladding layer, so that the thickness of the flat portion of the secondcladding layer can be controlled easily with high accuracy.

Further, the above-mentioned semiconductor laser has a barrier layerformed with a small refractive index and a large band gap as comparedwith the first and second cladding layers between the active layer andthe first cladding layer and/or between the active layer and the secondcladding layer. The beam divergence characteristic perpendicular to thejunction and the operating current characteristic are thus improved.

Furthermore, the semiconductor laser has a saturable optical absorbinglayer consisting of quantum well layers or a saturable optical absorbinglayer consisting of strained quantum well layers. In this way, the bandgap energy can be easily matched between the saturable optical absorbinglayer and the active layer.

In addition, the semiconductor laser according to the invention has anactive layer consisting of quantum well structure or strained quantumwell structure. This enables the self-sustained pulsation frequency tobe controlled, thereby producing a semiconductor laser of low noise.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a semiconductor laser according to afirst embodiment of the invention.

FIG. 2 is a graph showing the relation between the Al composition ratioand the γ value (the value related to a visibility: If γ=1, an outputlight is coherent. If γ=0, an output light is incoherent) and betweenthe layer thickness of the saturable optical absorbing layer and the γvalue according to a first embodiment.

FIG. 3 is a graph showing the relation between the Al composition ratioand the thickness of the saturable optical absorbing layer and theastigmatic distance according to the first embodiment.

FIG. 4 is a graph showing the relation between the Al composition thelayer thickness of the saturable optical absorbing layer and ratio andthe thickness of the saturable optical absorbing layer and the operatingcurrent according to the first embodiment.

FIG. 5 is a diagram showing the range of Al composition ratio u and thethickness of a saturable optical absorbing layer having a superiorcharacteristic according to the first embodiment.

FIG. 6 is a graph showing the relation between the thickness of thecladding layer and the γ value according to the first embodiment.

FIG. 7 is a graph showing the relation between the thickness of thecladding layer and the astigmatic distance according to the firstembodiment.

FIG. 8 is a graph showing the relation between the thickness of thecladding layer and the operating current according to the firstembodiment.

FIGS. 9(a), 9(b) and 9(c) are sectional views showing the fabricationprocess for a semiconductor laser according to the first embodiment.

FIG. 10 is a sectional view showing a semiconductor laser according to asecond embodiment of the invention.

FIGS. 11(a) and 11(b) are diagrams showing an energy band structure andthe refractive index in the neighborhood of an active layer according tothe second embodiment.

FIG. 12 is a graph showing the characteristic of beam divergence of thesecond embodiment and an example for comparison.

FIG. 13 is a graph showing the characteristic of the operating currentof the second embodiment and an example for comparison.

FIG. 14 is a graph showing the characteristic of a pulsation thresholdcurrent of the second embodiment and an example for comparison.

FIG. 15 is a graph showing the characteristic of characteristictemperature of the second embodiment and an example for comparison.

FIGS. 16(a) and 16(b) are sectional views showing the fabricationprocess for a semiconductor laser according to the second embodiment.

FIG. 17 is a graph showing the relation between the strain and the Gacomposition ratio of GaInP.

FIG. 18 is a sectional view showing a semiconductor laser according to athird embodiment of the invention.

FIG. 19 is an energy band diagram for a saturable optical absorbinglayer consisting of strained quantum well layers according to the thirdembodiment.

FIG. 20 is an energy band diagram for a strained MQW (Multi QuantumWell) active layer according to the third embodiment.

FIGS. 21(a) and 21(b) are graphs showing the optical power-currentcharacteristic for the third embodiment and an example for comparison.

FIG. 22 is a graph showing a threshold current and γ value against thenumber of well layers of the saturable optical absorbing layerconsisting of strained quantum well layers according to the thirdembodiment.

FIGS. 23(a), 23(b), 23(c), 23(d), 23(e) and 23(f) are sectional viewsshowing the fabrication process for a semiconductor laser according tothe third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described below with reference to the accompanyingdrawings showing embodiments.

FIRST EMBODIMENT

A schematic diagram of a sectional structure of a AlGaAs semiconductorlaser according to a first embodiment of the invention is shown inFIG. 1. In FIG. 1, reference numeral 1 designates an n-type GaAssemiconductor substrate, which has formed thereon an n-type A1_(xa)Ga_(1-xa) As cladding layer (typically having 1 to 2 μm in thickness and0.51 in composition ratio xa) 2a, an n-type A1_(u) Ga_(1-u) As firstsaturable optical absorbing layer having a thickness of S₁ (typicallyhaving a composition ratio of 0.12≦u≦0.14 and a carrier concentration of2×10¹⁷ to 5×10¹⁷ cm⁻³) 3, and an n-type A1_(xb) Ga_(1-xb) As claddinglayer (typically having a thickness of t₁ =0.3 μm and a compositionratio xb=0.51) 2b in that order. The two cladding layers 2a, 2b make upan n-type first cladding layer which has formed therein the firstsaturable optical absorbing layer 3.

The first cladding layer 2 (cladding layer 2b) has formed thereon anundoped A1_(q) Ga_(1-q) As active layer (typically having a filmthickness of 0.08 μm and a composition ratio q of 0.13) 4. The activelayer 4, in turn, has formed thereon a p-type A1_(ya) Ga_(1-ya) Ascladding layer (typically having a film thickness of t₂ of 0.3 μm and acomposition ratio ya of 0.51) 5a and a p-type A1_(u) Ga_(1-u) As secondsaturable optical absorbing layer (typically having a composition ratioof 0.12≦u≦0.14 and a carrier concentration of 4×10¹⁷ to 2×10¹⁸ cm⁻³) 6in that order. The second saturable optical absorbing layer 6 has formedon the central portion thereof a p-type A1_(yb) Ga_(1-yb) As claddinglayer (typically having a height of 0.5 to 1 μm, a width W of 4 μm ofthe lower side of the stripe, and a composition ratio yb of 0.51) 5b,which is a striped ridge section extending in the cavity direction. Thetwo cladding layers 5a, 5b constitute a p-type second cladding layer 5having a striped ridge section. The second cladding layer 5 includes thesecond saturable optical absorbing layer 6.

A p-type GaAs cap layer (typically having a thickness of 0.3 μm) 7 isformed on the upper surface of the ridge of the cladding layer 5b. Twon-type GaAs current-blocking layers (typically having a film thicknessof 0.8 μm) 8, 8 are formed on the side of the cap layer 7, the side ofthe cladding layer 5b, and the upper surface of the second saturableoptical absorbing layer 6 where no cladding layer 5b is formed. Thecurrent-blocking layers 8, 8 have the functions of narrowing current andabsorbing light. The cap layer 7 and the current-blocking layers 8, 8have a p-type GaAs contact layer (typically having a film thickness of 4μm) 9 formed thereon. The contact layer 9 has formed thereon a p-typeohmic electrode 10 made of Au--Cr, and the semiconductor substrate 1 hasan n-type ohmic electrode 11 made of Au--Sn--Cr formed on the lower sidethereof.

With this semiconductor laser according to the first embodiment, the γvalue, the astigmatic distance and the operating current for opticalpower of 3 mW were checked by changing the Al composition ratio u (thesame value) and the thickness (the same value for both, i.e., S₁ =S₂)between the first saturable optical absorbing layer 3 and the secondsaturable optical absorbing layer 6. In the case presented, a devicewith a cavity length of 250 μm is not coated at the edges.

The γ value represents the degree of self-sustained pulsation(interference between longitudinal modes). When γ is 1, theself-sustained pulsation does not occur, and the degree ofself-sustained pulsation increases with the approach of γ to zero.Experiments conducted by the inventors show that when a semiconductorlaser is mounted on an optical disk system, a satisfactory C/N isobtained for the γ value of 0.7 or less, and a very satisfactory valueis obtained for 0.6 or less. Also, as to the astigmatic distance in anoptical system used with an ordinary disk apparatus, a sufficient laserfocusing is attained with a satisfactory C/N for a small value of 17 μmor less, and an especially satisfactory C/N is obtained when theastigmatic distance is 15 μm or less where the laser beam issatisfactorily focused.

First, refer to FIG. 2 showing the relation between the γ value and thethickness of the first and second saturable optical absorbing layers 3,6 with the composition ratio u therebetween in the range of 0.12 to0.14. It is seen from FIG. 2 that the γ value is 0.7 or less for thethickness of more than 0.01 μm of the first and second saturable opticalabsorbing layers 3, 6, and 0.6 or less for the thickness of more than0.02 μm.

FIG. 3 shows the relation between the thickness of the first and secondsaturable optical absorbing layers 3, 6 and the astigmatic distance whenthe composition ratio u of the first and second saturable opticalabsorbing layers 3, 6 is in the range of 0.12 to 0.14. It will be seenfrom FIG. 3 that the astigmatic distance is less than 17 μm for thecomposition ratio of less than 0.13 of the first and second saturableoptical absorbing layers 3, 6 having a thickness of 0.01 μm or more, andthe astigmatic distance of less than 17 μm is obtained for thecomposition ratio u of 0.14 or less with the thickness of 0.02 μm ormore.

FIG. 4 shows the relation between the thickness of the first and secondsaturable optical absorbing layers 3, 6 and the operating current withoptical power of 3 mW when the composition ratio μ of the first andsecond saturable optical absorbing layers 3, 6 is in the range of 0.12to 0.14. The relation shown in FIG. 4 indicates that the operatingcurrent assumes a practically problem-free small value of less than 80mA for the thickness of 0.032 μm or less of the first and secondsaturable optical absorbing layers 3, 6 when the composition ratio is0.12 or more, for the thickness of 0.041 μm or less when the compositionratio is 0.13 or more, and for the thickness of 0.043 μm or less whenthe composition ratio is 0.14 or more.

As seen from FIGS. 2 to 4, it is possible to attain a satisfactory rangeof γ value, astigmatic distance and operating current as compared withthe prior art in a fundamental transverse mode for the first and secondsaturable optical absorbing layers 3, 6 with the difference of Alcomposition ratio with the active layer 4 in the range of -0.01 to 0.01(i.e., q-0.01≦u≦q+0.01; 0≦u<0.2). The reason for this satisfactoryeffect is that the difference in absolute value between the band gapenergy of the first and second saturable optical absorbing layers 3, 6and the band gap energy of the active layer 4, i.e., the energycorresponding to lasing wavelength is 0.0125 eV or less and thereforethe two energies are substantially equal to each other, therebyproducing a satisfactory saturable condition.

More specifically, even when the Al composition ratio q of the activelayer 4 is selected in the range of 0≦q<0.2 instead of limiting it to0.13, satisfactory values of γ, astigmatic distance and operatingcurrent are obtained with fundamental transverse mode as far as thevalues S₁ and S₂ of the thickness and the Al composition ratio u(0≦u<0.2) of the first and second saturable optical absorbing layers 3,6 are selected from within the range defined by hatching in FIG. 5.

In the example described above, the thickness t₁ of the cladding layer2b and the thickness t₂ of the cladding layer 5a are both 0.3 μm. The γvalue, the astigmatic distance and the operating current for the opticalpower of 3 mW were investigated by changing the two layer thicknesses t₁and t₂ to t. It is assumed that the composition ratio u of the first andsecond saturable optical absorbing layers 3, 6 is 0.13, the layerthickness is 0.03 μm, and the composition ratio q of the active layer 4is 0.13.

FIG. 6 shows the relation between the layer thickness t and the γ value.It is seen from FIG. 6 that the γ value is 0.7 or less for the layerthickness t of 0.27 μm or more, and 0.6 or less for the thickness of0.28 μm or more.

The relation between the layer thickness t and the astigmatic distanceis shown in FIG. 7. From FIG. 7, it is seen that the astigmatic distanceis 17 μm or less for the layer thickness t of 0.33 μm or less, and 15 μmor less for the thickness of 0.32 μm or less.

FIG. 8 shows the relation between the layer thickness t and theoperating current. It is seen from FIG. 8 that the operating currentbecomes a satisfactory value for the layer thickness t of 0.33 μm orless, and a very satisfactory value for the thickness of 0.32 μm orless.

As a result, the thickness t of the cladding layer 2b and the claddinglayer 5a can be selected within the range of 0.27 μm to 0.33 μm, orpreferably in the range of 0.28 μm to 0.32 μm. Although the claddinglayers 2b and 5a have been assumed to have the same thickness t (0.27μm≦t≦0.33 μm), the same effect is obtained also when the thicknesses t₁,t₂ of the cladding layers 2b, 5a are differentiated within the range of0.27 μm to 0.33 μm. An especially preferable effect is obtained when thethickness is selected in the range of 0.28 μm to 0.32 μm. Also when thecomposition ratio and the thickness of the first and second saturableoptical absorbing layers 3, 6 and the active layer 4 are changed, adesirable effect is obtained by optimizing the thickness of the claddinglayers 2b, 5a.

A method of fabricating a semiconductor laser according to the firstembodiment will be explained below with reference to FIG. 9.

First, as shown in FIG. 9(a), the metal organic chemical vapordeposition method (MOCVD method) or the molecular beam epitaxial method(MBE method) is used to grow an n-type cladding layer 2a, an n-typefirst saturable optical absorbing layer 3, an n-type cladding layer 2b,an undoped active layer 4, a p-type cladding layer 5a, a p-type secondsaturable optical absorbing layer 6, a p-type cladding layer 5b and ap-type cap layer 7 successively in that order on the n-typesemiconductor substrate 1.

Next, as shown in FIG. 9(b), a striped SiO₂ mask layer 12 having athickness of 0.2 μm is formed on the p-type cap layer 7 by the use of anordinary photolithography technique. With the SiO₂ layer 12 as a mask,an etching solution consisting of the H₃ PO₄ -H₂ O₂ -H₂ O system is usedto etch the p-type cap layer 7 and the p-type cladding layer 5b in sucha manner as to leave the p-type cladding layer 5b to a thickness of 0.1to 0.3 μm. After that, this residual cladding layer 5b is etched offwith a hydrochloric acid etchant to form a striped ridge section. In thecase of the hydrochloric acid etchant, since the A1_(v) Ga_(1-v) Assmall in Al composition ratio has an etching rate smaller than theA1_(s) Ga_(1-s) As large therein (v<s), the second saturabie opticalabsorbing layer 6 functions as what is called an etching stop layer atthe same time. Thus the etching can be stopped with a highcontrollability by the second saturable optical absorbing layer 6.

After that, as shown in FIG. 9(c), n-type current-blocking layers 8, 8are formed on the second saturable optical absorbing layer 6 and on theside of the p-type cladding layer 5b and the cap layer 7 having a ridgesection by the MOCVD or MBE method through the SiO₂ mask layer 12. Thenthe SiO₂ mask layer 12 is removed by the HF etchant to expose the p-typecap layer 7. A p-type contact layer 9 is then formed on the n-typecurrent-blocking layers 8, 8 and the p-type cap layer thus exposed bythe MOCVD or MBE method. An n-type ohmic electrode 11 made of Au--Sn--Crand a p-type ohmic electrode 10 made of Au--Cr are formed on the lowersurface of the n-type GaAs substrate 1 and on the upper surface of thep-type contact layer 9, thereby fabricating a semiconductor laser asshown in FIG. 1.

As described above, the second saturable optical absorbing layer 6,which has a band gap energy substantially equal to the energycorresponding to lasing wavelength (the A1_(v) Ga_(1-v) As has a largerband gap and a smaller refractive index with the increase in compositionratio v), is small in Al composition ratio (u<yb) and slow in etchingrate as compared with the cladding layer 5b and doesn't executeexcessive optical absorption, thereby making selection of sufficientlayer thickness possible for etching. As a consequence, the secondsaturable optical absorbing layer 6 functions satisfactorily as anetching stop layer in the mesa etching process described above. Thethickness of the cladding layer 5a providing a flat portion of thesecond cladding layer 5 on the active layer 4 can be controlled withhigh accuracy.

By the way in the case where a saturable optical absorbing layer havingsubstantially the same composition as the active layer is formed only onone of the cladding layers as according to the prior-art semiconductorlaser, it is necessary to increase the thickness of the flat portion(cladding layer 5a in FIG. 1) of the second cladding layer (5 in FIG. 1)having a ridge section, if self-sustained pulsation is to be maintained.The result is a larger astigmatic distance in the prior-artsemiconductor laser. The astigmatic distance can be reduced bydecreasing the thickness of the flat portion of the second claddinglayer. When the flat portion is reduced in thickness, however, thesaturable optical absorbing layer must be made thicker in order tomaintain the self-sustained pulsation, undesirably resulting in a largerasymmetry of the laser beam spot.

In the semiconductor laser with a self-sustained pulsation having asaturable optical absorbing layer, which executes optical absorption,the internal loss becomes large and threshold gain increases, as aresult, the operating current comes to be large. Accordingly, since itis thought that the provision of saturable optical absorbing layers inboth cladding layers as the present embodiment causes the operatingcurrent to increase remarkably, the concerned makers don't have a motiveto form the above configuration.

However, the following fact is made know by the present inventors. Inthe case where the saturable optical absorbing layers 3, 6 having a bandgap energy substantially equal to the energy corresponding to lasingwavelength are formed respectively in the first and second claddinglayers 2, 5, the layer thickness t₂ of the cladding layer 5a can bereduced, the resonant wave front in the direction perpendicular to thejunction is curved in the same manner as the wave front in the directionparallel to the junction and the traveling speed of these wave fronts isconsidered to approach each other, the astigmatic distance and the beamdivergence perpendicular to the junction become smaller than those ofthe prior-art semiconductor laser with a self-sustained pulsation.Accordingly, it is unnecessary to increase the thickness of thesaturable optical absorbing layer. Furthermore, since the layerthickness t₂ of the cladding layer 5a becomes small as described above,on the flat portion (the cladding layer 5a) the lateral beam divergenceof the current flowing from the ridge section (the cladding layer 5b) isreduced. As a result, a high current density is obtained with a smalloperating current, and the operating current can be reduced contrary tothe concerned makers' expectations.

The present embodiment employs an unprecedented configuration in whichthe second saturable optical absorbing the layer thickness of thesaturable optical absorbing layer and layer 6 is kept in contact withcurrent-blocking layers 8, 8 at the portion except the ridge section.This configuration makes the second saturable optical absorbing layer 6have the function as an etching stop layer. Accordingly, the layerthickness t₂ of the cladding layer 5a on the active layer 4 can becontrolled with high accuracy without forming an etching stop layerseparately.

Further, the second saturable optical absorbing layer 6 is arranged inproximity to the active layer 4, so it may function as a waveguidelayer. In the case where the second saturable optical absorbing layer 6is not in contact with the current-blocking layer 8, the transverse modewould be unstable. In spite of this, a stable transverse mode isattained according to the present embodiment since the second saturableoptical absorbing layer 6 is in contact with the current-blocking layer8 having such a composition as to cause light absorption. The reason whysuch a configuration is obtained is that the saturable optical absorbinglayers 3, 6 are formed in both the first and second cladding layers 2, 5and the thickness of the flat section is reduced.

According to the first embodiment having saturable optical absorbinglayers 3, 6 with a band gap energy substantially equal to the energycorresponding to lasing wavelength in both the first and second claddinglayers 2, 5, a satisfactory self-sustained pulsation is generated with asufficiently small astigmatic distance and a fundamental transverse modeas compared with the prior art through a sufficiently small operatingcurrent as compared with the prior art. As a result, the opticalfeedback induced noise is reduced and a highly reliable semiconductorlaser with a simple optical system structure is fabricated.

In addition, in the above configuration, it is to be desired that thedistance from the active layer 4 to the first saturable opticalabsorbing layer 3 is equal to that from the active layer 4 to the secondsaturable optical absorbing layer 6 in order to make more symmetricallaser beam spot.

In this first embodiment, it is desirable to insert an n-type GaAsbuffer layer between the semiconductor substrate 1 and the claddinglayer 2a.

The reason why the saturable optical absorbing layer can executesaturable optical absorption even if the band gap of it is larger thanthat of the active lasher is that it executes optical absorption betweenimpurity levels by impurity generating the conductivity thereof.

SECOND EMBODIMENT

Even when a saturable optical absorbing layer having a band gap ofenergy substantially equal to the energy corresponding to the lasingwavelength is formed in both the two cladding layers sandwiching theactive layer as according to the first embodiment, the beam divergence θin perpendicular direction (lamination direction) of the laser beamoutput cannot sometimes be sufficiently reduced. In the case where thebeam divergence θ in perpendicular direction is large, the couplingefficiency of an objective lens used in the optical disk system isreduced. It is therefore necessary to increase the laser output, therebyleading to the problem of a larger power consumption. One approach toreducing the perpendicular beam divergence θ is to reduce the differenceof refractive index between the cladding layers and the active layer.When the refractive index is reduced, however, the band gap differencebetween the cladding layers and the active layer is also reduced,thereby giving rise to the problem of a larger operating current.

The second embodiment described below is intended to obviate theabove-mentioned problems. According to the second embodiment, a barrierlayer having a refractive index smaller than and a band gap larger thanthe first and second cladding layers is formed between the firstcladding layer and the active layer and/or between the second claddinglayer and the active layer. The second embodiment will be explainedbelow.

FIG. 10 is a schematic diagram showing a sectional structure of anAlGaAs semiconductor laser according to the second embodiment of theinvention. In FIG. 10, numeral 21 designates an n-type GaAssemiconductor substrate, which has formed thereon an n-type GaAs bufferlayer (typically having a thickness of 0.5 μm) 22, an n-type A1_(xa)Ga_(1-xa) As cladding layer (typically having a thickness of 1.5 μm anda composition ratio xa of 0.5) 23a, an n-type A1_(ua) Ga_(1-ua) As firstsaturable optical absorbing layer (typically having a thickness of 0.03μm and a composition ratio ua of 0.13) 24 and an n-type A1_(xb)Ga_(1-xb) As cladding layer (typically having a thickness of 0.2 μm anda composition ratio xb of 0.5) 23b, in that order. The two claddinglayers 23a, 23b make up an n-type first cladding layer 23, in which thefirst saturable optical absorbing layer 24 is formed.

The first cladding layer 23 (cladding layer 23b) has formed thereon ann-type A1_(za) Ga_(1-za) As first barrier layer (typically having athickness of 0.1 μm and a composition ratio za of 0.55) 25, an undopedA1_(q) Ga_(1-q) As active layer (typically having a thickness of 0.08 μmand a composition ratio q of 0.13) 26 and a p-type A1_(zb) Ga_(1-zb) Assecond barrier layer (typically having a thickness of 0.1 μm and acomposition ratio zb of 0.55) 27, in that order.

The second barrier layer 27, on the other hand, has formed thereon ap-type A1_(ya) Ga_(1-ya) As cladding layer (typically having a thicknessof 0.2 μm and a composition ratio ya of 0.5) 28a and a p-type A1_(ub)Ga_(1-ub) As second saturable optical absorbing layer (typically havinga thickness of 0.03 μm and a composition ratio ub of 0.13) 29, in thatorder. The second saturable optical absorbing layer 29, in turn, has thecentral portion thereof formed with a p-type A1_(yb) Ga_(1-yb) Ascladding layer (typically having a thickness of 0.7 μm, a width W of 3.5μm of the lower side of the stripe and a composition ratio yb of 0.5)28b formed as a striped ridge section extending in the cavity direction.The two cladding layers 28a, 28b constitute a p-type second claddinglayer 28 having a striped ridge section. The second saturable opticalabsorbing layer 29 is formed in the second cladding layer 28.

The following relation holds between the A1 composition ratio of eachlayer:

ua<xa, xb, ya, yb: ub<xa, xb, ya, yb:

za>xa, xb, ya, yb: zb>xa, xb, ya, yb:

q<xa, xb, ya, yb:

The cladding layer 28b has the upper surface of the ridge sectionthereof formed with a p-type GaAs cap layer typically having a thicknessof 0.3 μm) 30. Two n-type GaAs current-blocking layers (typically havinga thickness of 0.8 μm) 31, 31 are formed on the side of the cap layer30, on the side of the cladding layer 28b, and on the upper surface ofthe second saturable optical absorbing layer 29 where no cladding layer28b is formed. The cap layer 30 and the current-blocking layers 31, 31have formed thereon a p-type GaAs contact layer (typically having athickness of 6 μm) 32. The contact layer 32, in turn, has formed thereona p-type ohmic electrode 33 of Au--Cr, and the lower side of thesemiconductor substrate 21 has formed thereon an n-type ohmic electrode34 of Au--Sn--Cr.

FIG. 11(a) is a schematic diagram showing the energy band structure inthe neighborhood of the active layer 26 of a semiconductor laseraccording to the second embodiment, and FIG. 11(b) is a schematicdiagram showing the refractive index in the neighborhood of the activelayer 26. As seen from these diagrams, the first and second claddinglayers 23, 28 have a refractive index smaller than and a band gap largerthan the active layer 26. First and second barrier layers 25, 27 havinga refractive index smaller than and a band gap larger than the first andsecond cladding layer 23, 28 and a thickness larger than the activelayer 26 are formed in such a position as to sandwich the active layer26 on both sides thereof. The resulting assembly is sandwiched by thefirst and second saturable optical absorbing layers 24, 29 having thesame band gap (refractive index) as the active layer 26, i.e., having anenergy band gap equal to the energy corresponding to raising wavelength,which layers 24, 29 are formed in the first and second cladding layer23, 28.

The foregoing description involves only a configuration including thefirst and second barrier layers 25, 27 on both sides of the active layer26. Instead, only the second barrier layer 27 may be formed without thefirst barrier layer 25. The beam divergence perpendicular to thejunction θ (degree) of laser beam with output power of 3 mW at 25° C.,the operating current I_(op) (mA), the pulsation threshold currentI_(th) (mA) at temperature of 25° C. and the characteristic temperatureT_(o) (K) in the range of 25° C. to 70° C. were measured with regard toa semiconductor laser as case 1 of the second embodiment having thefirst and second barrier layers 25, 27 on both sides thereof, asemiconductor laser as case 2 of the second embodiment having the sameconfiguration as case 1 except that the first barrier layer 25 isomitted, and a semiconductor laser as a reference case lacking both thefirst and second barrier layers 25, 27. The lasers according to thecases 1 and 2 and the reference case have the same distance between thefirst saturable optical absorbing layer 24 and the active layer 26 andbetween the second saturable optical absorbing layer 29 and the activelayer 26, and other conditions are also shared by them, except for thepresence or absence of the first and second barrier layers. Also, themeasurement was conducted on two common values of 3×10¹⁷ cm³ and 6×10¹⁷cm³ of the p-type carrier concentration between the second barrier layer27 and the cladding layer 28a. The result is shown in FIGS. 12 to 15. Inthis measurement, each laser is subjected to a satisfactoryself-sustained pulsation in unit transverse mode.

FIG. 12 shows the relationship between the beam divergence perpendicularto the junction θ (degree) of a semiconductor laser as cases 1 and 2 anda reference case and the p-type carrier concentration. It is seen fromFIG. 12 that regardless of the carrier concentration of the secondbarrier layer 27 and the cladding layer 28a the beam divergence θ ofcases 1 and 2 and the reference case is about 33, about 35 and about 37degrees, respectively. More specifically, the beam divergence θ issmaller for the case 2 having the second barrier layer 27 than for thereference case lacking both the first and second barrier layers 25, 27,and is even smaller for the case 1 having both the first and secondbarrier layers 25, 27.

FIG. 13 is a diagram showing the relationship between the operatingcurrent I_(op) (mA) of a semiconductor laser as cases 1 and 2 and areference case and the p-type carrier concentration. As will be seenfrom FIG. 13, the operating current I_(op) is smaller for case 2 havingthe second barrier layer 27 than for the reference case lacking both thefirst and second barrier layers 25, 27, and is even smaller for the case1 having both the first and second barrier layers 25, 27. Especially, itis noted that the operating current is smaller when the carrierconcentration of the second barrier layer 27 and the cladding layer 28ais reduced.

The relationship between the threshold current I_(th) (mA) of asemiconductor laser as cases 1 and 2 and a reference case and the p-typecarrier concentration is shown in FIG. 14. It is noted from FIG. 14 thatthe threshold current I_(th), like the operating current I_(op), issmaller for the case 2 having the second barrier layer 27 than for thereference case lacking both the first and second barrier layers 25, andis even smaller for the case 1 having both the first and second barrierlayers 25, 27. Further, the threshold current can be reduced to asmaller level when the carrier concentration of the second barrier layer27 and the cladding layer 28a is reduced.

FIG. 15 is a diagram showing the relationship between the characteristictemperature T_(o) (K) of a semiconductor laser as cases 1 and 2 and areference case and the p-type carrier concentration. It is seen fromFIG. 15 that the characteristic temperature T_(o) is higher for the case2 having the second barrier layer 27 than for the reference case lackingboth the first and second barrier layers 25, 27, and is even higher forthe case 1 having both the first and second barrier layers 25, 27.Especially, it is noted that the characteristic temperature can beincreased considerably when the carrier concentration of the secondbarrier layer 27 and the cladding layer 28a is large.

FIGS. 12 to 15 indicate that the beam divergence perpendicular to thejunction θ and the operating current I_(op) are better for the case 2having the second barrier layer 27 than for the reference case lackingboth the first and second barrier layers 25, 27, and also that it ismuch better for the case 1 having both the first and second barrierlayers 25, 27. As will be seen from the cases 1 and 2, the thresholdcurrent I_(th) and the characteristic temperature T_(o) assume a valuewhich poses no problem at all as long as the p-type carrierconcentration of the second barrier layer 27 and the cladding layer 28ais in the normal range. As described above, with the increase in thep-type carrier concentration, the characteristic temperature T_(o)remains satisfactory while the operating current I_(op) and thethreshold current I_(th) are deteriorated. As the p-type carrierconcentration decreases, on the other hand, the characteristictemperature T_(o) is deteriorated while both the operating temperatureI_(op) and the threshold current I_(th) are improved. These trendsshould be selectively used in the particular application. Further,though not shown, the astigmatic distance remains substantially the samefor the reference case and the case 2, and is smaller for the case 1.The carrier concentration of other than the second barrier layer 27 andthe cladding layer 28a, which has no substantial effect on the lasercharacteristics, may be selected appropriately within normal range.

The operating current and the beam divergence perpendicular to thejunction θ can be reduced even when the thickness of the first andsecond barrier layers 25, 27 is smaller than that of the active layer26. However, this effect is more conspicuous especially, in the casewhere the thickness of the first and second barrier layers 25, 27 islarger than that of the active layer 26 than in the case where thethickness of the first and second barrier layers 25, 27 is reducedconsiderably (i.e., with the light beam not substantially affected bythe first and second barrier layers 25, 27). Such a conspicuous effectis obtained in the case where the first and second saturable opticalabsorbing layers 24, 29 are configured in such a position as to sandwichthe active layer 26 on the one hand and the first barrier layer 25 orthe second barrier layer 27 having a larger thickness than the activelayer 26 is formed in proximity to the active layer 26 on the otherhand. An especially desirable effect is obtained in the case where thefirst and second saturable optical absorbing layers 24, 29 and the firstand second barrier layers 25, 27 are positioned symmetrically withrespect to the active layer 26.

A method of fabricating a semiconductor laser according to the secondembodiment will be explained below with reference to FIG. 16.

First, as shown in FIG. 16(a), an n-type buffer layer 22, an n-typecladding layer 23a, an n-type first saturable optical absorbing layer24, an n-type cladding layer 23b, an n-type first barrier layer 25, anundoped active layer 26, a p-type second barrier layer 27, a p-typecladding layer 28a, a p-type second saturable optical absorbing layer29, a p-type cladding layer 28b and a p-type cap layer 30 are formedsuccessively in that order on an n-type semiconductor substrate 21 bythe MOCVD or MBE method. After that, a striped SiO₂ mask layer 35 havinga thickness of 0.2 μm is formed on the p-type cap layer 30 by the use ofordinary photolithography or the like. The p-type cap layer 30 and thep-type cladding layer 28b are etched off in such a manner as to leavethe p-type cladding layer 28b in the thickness of 0.1 to 0.3 μm by theetching solution consisting of the H₃ PO₄ -H₂ O₂ -H₂ O system with theSiO.sub. 2 mask layer 35. Then the remaining cladding layer 28b isetched off by the hydrochloric acid etchant to form a striped ridgesection. In the process, the second saturable optical absorbing layer 29functions as what is called an etching stop layer so that the mesaetching may be stopped with high controllability at the second saturableoptical absorbing layer 29 as in the first embodiment described above.

After that, as shown in FIG. 16(b), the n-type current-blocking layers31, 31 are formed on the side of the p-type cladding layer 28b and thep-type cap layer 30 shaped as a ridge as well as on the second saturableoptical absorbing layer 29 by the MOCVD or MBE method with the SiO₂ masklayer 35. In the next step, the SiO₂ mask layer 35 is removed by the HFetchant to expose the p-type cap layer 30, after which a p-type contactlayer 32 is formed on the exposed p-type cap layer 30 and the n-typecurrent-blocking layers 31, 31 by the MOCVD or MBE method. Then a p-typeohmic electrode 33 of Au--Cr and an n-type ohmic electrode 34 ofAu--Sn--Cr are formed on the upper surface of the p-type contact layer32 and on the lower side of the n-type GaAs substrate 21, therebyfabricating a semiconductor laser as shown in FIG. 10.

In the semiconductor laser according to the second embodiment, besidesthe configuration of the first embodiment, the barrier layers 25, 27having a refractive index smaller than and a band gap larger than thefirst and second cladding layers 23, 26 are formed between the firstcladding layer 23 and the active layer 26 and/or between the activelayer 26 and the second cladding layer 28, thereby reducing the opticalfeedback induced noise sufficiently. Further, this configuration reducesthe operating current for a reduced power consumption on the one handand reduces the beam divergence perpendicular to the junction for animproved coupling coefficient with an objective lens which may be usedin combination with the particular laser on the other hand.Consequently, power consumption is reduced even more. Further, thesmaller astigmatic distance sufficiently converges the laser beamoutput, thereby improving the S/N for crosstalks when the laser is usedas a light source of an optical disk system or the like.

According to the second embodiment, the barrier layer of thicknesslarger than active layer is further formed in the configuration in whichthe saturable optical absorbing layers are formed in both claddinglayers, so, even in the configuration affected by the difference inrefractive index between the active layer and the barrier layer, thebeam divergence perpendicular to the junction can be reduced. Thus, whenthe barrier layer is thick, the variation in the beam divergenceperpendicular to the junction can be reduced and the variation in thethickness of the barrier layer has little effect on the lasercharacteristics, thereby improving the production yield.

Although the Al_(q) Ga_(1-q) As (0≦q<0.2) layer is used as the activelayers 4, 26 according to the first and second embodiments, a quantumwell structure as described in the third embodiment later is of courseusable as an alternative with equal effect.

Further, instead of the AlGaAs semiconductor laser described aboveaccording to the first and second embodiments, a similar effect isattained with an A1GaInP semiconductor laser, for example. The A1GaInPsemiconductor, which decreases in refractive index and increases in bandgap with the increase in the A1 composition ratio like the A1GaAssemiconductor, enables the band gap and the refractive index to beselected by changing the A1 composition ratio.

THIRD EMBODIMENT

With reference to the first and second embodiments, in the case wherethe saturable optical absorbing layer is of a bulk structure (with thelayer thickness more than several hundred Å where the quantum effect isnot caused), the band gap energy of the saturable optical absorbinglayer can be made substantially equal to the energy corresponding tolasing wavelength by changing the composition ratio of the saturableoptical absorbing layer and by thus regulating the magnitude of the bandgap. When the composition ratio of the saturable optical absorbing layeris changed in this manner, however, a crystalline defect occurs in thecrystal of the saturable optical absorbing layer of the A1GaInP orGaInAsP semiconductor laser, with the result that a high thresholdcurrent deteriorates the characteristics of the semiconductor laser.Especially when the lasing wavelength is short (as when the active layerconsists of MQW structure), it is difficult to make a band gap energysubstantially equal to the energy corresponding to lasing wavelength. Inother words, the band gap energy of the saturable optical absorbinglayer is difficult to control merely with the composition ratio.

Also, the semiconductor laser with a self-sustained pulsation having anactive layer of MQW structure has the problem that the astigmaticdistance is increased or the kink (nonlinearity of the outputpower-current characteristic) is caused at low output power.

When the saturable optical absorbing layer is of a quantum wellstructure with a quantum well layer having a layer thickness of 200 Åless, control of the thickness of the well layer (hereinafter referredto as "the well width") in the above-mentioned range makes it possibleto match the band gap energy easily between the saturable opticalabsorbing layer and the active layer without posing any problem ofcrystalline defect or the like. When the well width is reduced, forexample, the band gap energy is increased. An increased well width, onthe other hand, decreases the band gap energy. The number of welllayers, which is determined by the material used and the well width, maybe single or plural.

The strained quantum well structure of the saturable optical absorbinglayer facilitates the matching of the band gap energy between thesaturable optical absorbing layer and the active layer with highaccuracy by a method of introducing a lattice strain by changing thecomposition ratio of the compound semiconductor used for the well layersand by the above method of controlling the well width. Also, as in thecase where the saturable optical absorbing layer is constructed of aquantum well structure, the number of well layers is determined by thematerial used and the well width and may be single or plural.

FIG. 17 is a graph showing the relation between the Ga content x and thestrain in the Ga_(x) In_(1-x) P formed on a GaAs semiconductorsubstrate. In the case where the strain assumes a positive value, acompression strain is indicated, while a negative value represents atensile strain. In this case, if the compression strain is increased,i.e., the Ga content is reduced, the lasing wavelength becomes longer.An increased tensile strain, i.e., an increased Ga content, on the otherhand, shortens the lasing wavelength. As shown in FIG. 17, the Gacontent and the strain have a linear relation with each other. Bychanging the Ga content, therefore, the desired strain is easilyobtained, thereby making it possible to control the band gap energy ofthe saturable optical absorbing layer. Also, as long as the well widthis kept below a critical layer thickness (the layer thickness where acrystalline defect begins to occur) of the desired strain, the problemof crystalline defect is not posed.

Once the active layer is constructed of a quantum well structure or astrained quantum well structure, the frequency of the self-sustainedpulsation can be controlled to produce a low-noise semiconductor laserin addition to the effect of producing a quantum well structure or astrained quantum well structure of a saturable optical absorbing layer.Although the operating current can be reduced by decreasing the numberof well layers, the gain is reduced. Therefore, the number of welllayers is determined in combination with the well width.

Now, a third embodiment of the invention configured as described abovewill be explained.

FIG. 18 is a schematic diagram showing a sectional structure of anA1GaInP semiconductor laser according to a third embodiment of theinvention, as related to a red semiconductor laser. In FIG. 18, numeral41 designates an n-type GaAs semiconductor substrate, on which areformed an n-type GaInP buffer layer (typically having a thickness of 0.3μm) 42, an n-type (A1₀.7 Ga₀.3)₀.5 In₀.5 P cladding layer 43a, an n-typestrained quantum well first saturable optical absorbing layer 44 and ann-type (A1₀.7 Ga₀.3)₀.5 In₀.5 P cladding layer 43b, in that order. Thecladding layers 43a, 43b make up a first cladding layer (typicallyhaving a thickness of 0.8 μm) 48, which includes the first saturableoptical absorbing layer 44 therein.

The first cladding layer 43 (cladding layer 43b) has formed thereon anundoped strained MQW active layer 45. The active layer 45, in turn, hasformed thereon a p-type (A1₀.7 Ga₀.3)₀.5 In₀.5 P cladding layer 46a, ap-type strained quantum well second saturable optical absorbing layer 47and a p-type (A1₀.7 Ga₀.3)₀.5 In₀.5 P cladding layer 46b constituting astriped ridge section extending in the cavity direction, in that order.The two cladding layers 46a, 46b make up a p-type second cladding layer(typically having a thickness of 1.1 μm and a stripe width of 5 μm) 46having a striped ridge section. The second cladding layer 46 has formedtherein the second saturable optical absorbing layer 47.

The cladding layer 46b has the upper surface of the ridge sectionthereof formed with a p-type GaInP cap layer 48. Two n-type GaAscurrent-blocking layers (typically having a thickness of 1 μm) 49, 49are formed on the side of the cap layer 48, on the side of the claddinglayer 46b, and on the upper surface of the second saturable opticalabsorbing layer 47 lacking the cladding layer 46b. The cap layer 48 andthe current-blocking layers 49, 49 have formed thereon a p-type GaAscontact layer 50. The contact layer 50 has formed thereon a p-type ohmicelectrode 51 of Au--Cr, and the semiconductor substrate 41 has formed onthe lower side thereof an n-type ohmic electrode 52 of Au--Sn--Cr.

The energy band of the first and second saturable optical absorbinglayers 44, 47 is shown in FIG. 19. Each of the saturable opticalabsorbing layers 44, 47, as shown in FIG. 19, includes a barrier layer(typically having a thickness of 50 Å) 53 of (A1₀.7 Ga₀.3)₀.5 In₀.5 Pand a well layer (typically having a thickness of 100 Å strain of +0.5to +1.0%) 54 of Ga_(x) In_(1-x) P alternately with each other, therebyforming three well layers 54 according to this embodiment.

The energy band of the strained MQW active layer 45 is shown in FIG. 20.The active layer 45, as shown in FIG. 20, includes a guide layer(typically having a thickness of 500 Å) 55 of (A1₀.5 Ga₀.5)₀.5 In₀.5 P,which in turn has formed thereon a barrier layer (typically having athickness of 50 Å) 56 of (A1₀.5 Ga₀.5)₀.5 In₀.5 P and a well layer(typically having a thickness of 100 Å and a strain of +0.5%) 57 ofGaInP in alternate layers. Further, a guide layer (typically having athickness of 500 Å) 55 of (A1₀.5 Ga₀.5)₀.5 In₀.5 P is formed on theresulting assembly. According to this embodiment, five to eight welllayers 57 produce a superior characteristic. In FIG. 20, as an example,five well layers 57 are formed. The guide layers 55 are formed for thepurpose of improving the luminous efficiency by trapping and guiding thelight.

FIG. 21 is a graph showing the output power-current characteristic of aprior-art semiconductor laser with a self-sustained pulsation and asemiconductor laser according to the present embodiment with thethickness of the active layers and the cladding layers appropriatelyselected. FIG. 21(a) shows the case of a prior-art semiconductor laser,and FIG. 21(b) the case of a semiconductor laser according to theembodiment. The semiconductor laser used according to the embodimentincludes a strained MQW active layer 45 with five well layers 57 havinga strain of +0.5%, strained quantum well saturable optical absorbinglayers 44, 47 with a well layer 54 having a strain of +0.6%, and acladding layer 46a having a thickness of 0.25 μm under thecurrent-blocking layer 49. The prior-art semiconductor laser thus usedincludes a strained MQW active layer composed of seven well layers (100Å) having a strain of 0.5% and six barrier layers (50 Å), with thecladding layer as thick as 0.35 μm under the current-blocking layer. Itis seen from FIGS. 21(a) and 21(b) that according to the embodimentunder consideration, the nonlinearity of the output power-currentcharacteristic is improved considerably. The threshold current,astigmatic distance and the presence or absence of kink in thesesemiconductor lasers are also shown in Table 1. As obvious from Table 1,the astigmatic distance is shortened according to the embodiment.

                  TABLE 1                                                         ______________________________________                                               Threshold                                                                             Astigmatic                                                                              Presence                                                    current distance  or absence                                                  (mA)    (μm)   of kink                                              ______________________________________                                        Prior art                                                                              50˜55                                                                             15˜20                                                                             With kink at 3 to 4 mW                           Embodiment                                                                             52˜56                                                                             10˜15                                                                             No kink up to 5 mW                               ______________________________________                                    

FIG. 22 is a graph showing the γ value representing the lightinterference and the threshold current against the number of the welllayers 54 of the strained quantum well saturable optical absorbinglayers 44, 47 according to the embodiment. When the γ value is 1.0, theoperation is in single mode. The smaller the γ value, the stronger theself-sustained pulsation. For the γ value less than 0.7, a satisfactoryself-sustained pulsation is obtained. The semiconductor laser used inthis experiment includes a well layer 54 having a thickness of 100 Å anda strain of +0.6%, a well layer 57 having a thickness of 100 Å, abarrier layer 56 having a thickness of 50 Å and a strain of +0.5%, and acladding layer 46a under the current-blocking layer 49 having athickness of 0.25 μm. In the case where the number of the well layers 54is increased in order to reduce the light interference, the thresholdcurrent is increased. As a result, according to the embodiment, it isdesirable to form one to three well layers 54 in the strained quantumwell saturable optical absorbing layers 44, 47.

A method of fabricating a semiconductor laser according to the thirdembodiment will be described below with reference to FIG. 23.

First, an n-type buffer layer 42, an n-type cladding layer 43a, ann-type strained quantum well saturable optical absorbing layer 44, ann-type cladding layer 43b, an undoped strained MQW active layers 45, ap-type cladding layer 46a, a p-type strained quantum well saturableoptical absorbing layer 47, a p-type cladding layer 46b and a p-type caplayer 48 are formed sequentially on an n-type semiconductor substrate 41by the MOCVD method (FIG. 23(a)). Then, an SiO₂ film is formed by theelectron beam vapor deposition or the CVD method, and a mask 58 isproduced by patterning into a stripe about 5 μm wide by photolithography(FIG. 23(b)). That portion of the cap layer 48 and the cladding layer48b which is not covered by the mask 58 is removed into a ridge shape byetching (30° C., 30 seconds) using hydrobromic acid through the mask 58(FIG. 23(c)).

After that, an n-type GaAs is grown selectively thereby to formcurrent-blocking layers 49, 49 (FIG. 23(d)). After removing the mask 58by a buffer hydrofluoric acid solution, a contact layer 50 of p-typeGaAs is formed by the MOCVD method (FIG. 23(e)). A p-type ohmicelectrode 51 is formed on the upper surface of the contact layer 50 andan n-type ohmic electrode 52 on the lower surface of the substrate 41,thereby fabricating a semiconductor laser (FIG. 23(f)).

Although the third embodiment uses a strained quantum well saturableoptical absorbing layer having a strained quantum well structure, aquantum well saturable optical absorbing layer having a quantum wellstructure may bee used as an alternative with equal effect. The energycorresponding to lasing wavelength is desirably substantially equal tothe quantization level of the conduction band and the valence band inthe well layer of the quantum well structure constituting the saturableoptical absorbing layer. The third embodiment is applicable to asemiconductor laser having an active layer of MQW, SQW (Single QuantumWell) or bulk structure as well as the strained MQW structure.

The third embodiment has been explained above with reference to a redsemiconductor laser using GaInP and A1GaInP. The embodiment, however, isof course applicable to all semiconductor lasers with a self-sustainedpulsation using semiconductors of other compound semiconductors.

It will thus be understood from the foregoing description that in thesemiconductor laser according to the third embodiment, the band gapenergy can be easily matched between the saturable optical absorbinglayer and the active layer by constructing the former as a quantum wellstructure. Further, with a strained quantum well structure of thesaturable optical absorbing layer, the band gap energy can be controlledeasily and accurately, thereby realizing a semiconductor laser having asuperior low-noise high-output characteristic.

Since the Al composition ratio of the cladding layer 46b is differentfrom that of the quantum well layer 57 of the saturable opticalabsorbing layer 47 having a quantum well structure or a strained quantumwell structure, the saturable optical absorbing layer 47 can function asan etching stop layer by using a selective etchant wherein the etchingrate for compound semiconductor with lower Al composition ratio issmaller than that for compound semiconductor with higher Al compositionratio.

That is, as well as the above first, second and third embodiments, in asemiconductor laser with a self-sustained pulsation which consists ofcompound semiconductor containing Al such as AlGaAs, AlGaInP or the likeand has a saturable optical absorbing layer with a band gap energysubstantially equal to the energy corresponding to lasing wavelength,the saturable optical absorbing layer includes at least a portion withAl composition layer lower than the cladding layer. Consequently, byusing a selective etchant wherein the etching rate for compoundsemiconductor with lower Al composition ratio is smaller than that forcompound semiconductor with higher Al composition ratio, this saturableoptical absorbing can function as an etching stop layer.

In each of the above-mentioned embodiments, the place where a saturableoptical absorbing layer is formed is determined by the required amountof light absorption and is not limited to the interior of the twocladding layers. A saturable optical absorbing layer may be formed, forexample, on the upper surface of the first cladding layer and the lowersurface of the second cladding layer.

Although each cladding layer constituting the first and second claddinglayers is configured to have the same refractive index and band gap ineach of the embodiments described above, they are not necessarilyidentical to each other. In other words, the first and second claddinglayers are not required to be uniform.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined b the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A method for fabricating a semiconductor laserwith a self-sustained pulsation, said laser having an active layer and apredefined lasing wavelength, comprising the steps of:forming byepitaxial growth on a semiconductor substrate of a first conductive typein this order, a first cladding layer of the first conductive type, afirst saturable optical absorbing layer of the first conductive type, asecond cladding layer of the first conductive type, a first barrierlayer of the first conductive type, an active layer, a second barrierlayer of a second conductive type opposite to the first conductive type,a third cladding layer, a second saturable optical absorbing layer ofthe second conductive type, a fourth cladding layer of the secondconductive type, and a cap layer of the second conductive type; each ofsaid first to fourth cladding layers having a refractive index smallerthan a band gap larger than said active layer and said saturable opticalabsorbing layers, each of said saturable optical absorbing layers havinga band gap of energy substantially equal to the energy corresponding tolasing wavelength, and each of said barrier layers having a refractiveindex smaller than and a band gap larger than said second and thirdcladding layers; forming a striped mask pattern on said cap layer;etching off a part of said cap layer and said fourth cladding layer witha formed mask pattern as a mask, thereby forming a striped ridgesection; and forming a current-blocking layer of the first conductivetype on the upper surface of said second saturable optical absorbinglayer where the part of said fourth cladding layer is etched off and onthe sides of said cap layer and said fourth cladding layer having astriped ridge shape.
 2. A method for fabricating a semiconductor laserwith a self-sustained pulsation, said laser having an active layer,comprising the steps of:forming a first cladding layer of a firstconductive type on a semiconductor substrate of the first conductivetype; forming an active layer on said first cladding layer; forming onsaid active layer a second cladding layer of a second conductive typeopposite to the first conductive type; and forming in at least one ofsaid first cladding layer and said second cladding layer a saturableoptical absorbing layer having a structure which is one of a quantumwell structure and a strained quantum well structure which has a bandgap of energy substantially equal to the energy corresponding to lasingwavelength; each of said first and second cladding layers having arefractive index smaller than and a band gap larger than said activelayer, and, said saturable optical absorbing layer having a band gapsmaller than and a refractive index larger than said first and secondcladding layers.
 3. A method for fabricating a semiconductor laser witha self-sustained pulsation according to claim 2;wherein said saturableoptical absorbing layer is formed so as to function as an etching stoplayer.
 4. A method for fabricating a semiconductor laser with aself-sustained pulsation according to claim 2;wherein said secondcladding layer has a flat section and a striped ridge section, and saidsaturable optical absorbing layer is formed between the flat section andthe striped ridge section.
 5. A method for fabricating a semiconductorlaser with a self-sustained pulsation according to claim 2;wherein saidactive layer has a structure which is one of a quantum well structureand a strained quantum well structure.
 6. A method for fabricating asemiconductor laser with a self-sustained pulsation according to claim2, further comprising the step of;forming a barrier layer, which has arefractive index smaller than and a band gap larger than said first andsecond cladding layers, between at least one pair of (i) said firstcladding layer and said active layer and (ii) said active layer and saidsecond cladding layer.
 7. A method for fabricating a semiconductor laserwith a self-sustained pulsation, said laser having an active layer and apredefined lasing wavelength, comprising the steps of:forming byepitaxial growth on a semiconductor substrate of a first conductive typein this order, a cladding layer of the first conductive type, an activelayer, a first cladding layer of a second conductive type opposite tothe first conductive type, a saturable optical absorbing layer of thesecond conductive type, a second cladding layer of the second conductivetype, and a cap layer of the second conductive type; each of saidcladding layers having a refractive index smaller than and a band gaplarger than said active layer and said saturable optical absorbinglayer, and said saturable optical absorbing layer having a band gap ofenergy substantially equal to the energy corresponding to the lasingwavelength, forming a striped mask pattern on said cap layer; etchingoff a part of said cap layer and said second cladding layer of thesecond conductive type with a formed mask pattern as a mask, therebyforming a striped ridge section; and forming a current-blocking layer ofthe first conductive type on the upper surface of said saturable opticalabsorbing layer where the part of said second cladding layer of thesecond conductive type is etched off and on the sides of said cap layerand said second cladding layer of the second conductive type having astriped ridge shape.
 8. A method for fabricating a semiconductor laserwith a self-sustained pulsation according to claim 7;wherein saidsaturable optical absorbing layer is formed so as to function as anetching stop layer.
 9. A method for fabricating a semiconductor laserwith a self-sustained pulsation according to claim 7, further comprisingthe step of:forming in said cladding layer of the first conductive typea saturable optical absorbing layer having a band gap of energysubstantially equal to the energy corresponding to the lasingwavelength.
 10. A method for fabricating a semiconductor laser with aself-sustained pulsation according to claim 7, further comprising thestep of:forming a barrier layer, which has a refractive index smallerthan and a band gap larger than said cladding layer of the firstconductive type and said first cladding layer of the second conductivetype, between at least one pair of (i) said cladding layer of the firstconductive layer and said active layer and (ii) said active layer andsaid first cladding layer of the second conductive type.